Simultaneous channel estimation of a carrier and an interferer

ABSTRACT

Interference suppression in a receiver in a wireless network that utilizes training sequences for synchronization and channel estimation, wherein the training sequence of an interfering channel overlaps the training sequence of a desired channel to cause degraded channel estimation, by generating a channel estimate for a carrier part of a received signal; generating a residual signal where the carrier part has been removed from the received signal; generating covariance matrix estimates for interferer channel estimate candidates; selecting carrier and interferer channel estimates having the lowest energy in the covariance matrix; and, explicitly generating the selected interferer channel estimate.

BACKGROUND OF THE INVENTION

The present invention relates generally to digital receivers operable toreceive a receive signal transmitted upon a channel susceptible tointerference, such as co-channel interference. More particularly, thepresent invention relates to apparatus, and an associated method, foridentifying an interfering-signal component portion of the signalreceived by the receiver and for selectively suppressing theinterfering-signal component portion out of the receive signal.

The interfering-signal component portion is identified even withoutprior knowledge of a training sequence associated with, and forming aportion of, the interfering signal component portion. The receive signalis analyzed, first to identify a training sequence associated with aninterfering signal component portion. And, a determination is made ofthe manner by which to recover best the wanted-signal component of thereceive signal. A selection is made either to recover the wanted-signalcomponent of the receive signal, e.g., by jointly detecting aninterfering-signal component portion together with the wanted-signalcomponent and thereby suppressing the interfering-signal componentportion, or to recover the wanted-signal component by detecting merelythe wanted-signal component of the receive signal.

When embodied in a cellular communication system, operation of anembodiment of the present invention facilitates better suppression ofco-channel interference. Because co-channel interference is better ableto be suppressed, channels defined in a cellular communication systemcan be reused in a more efficient manner, thereby to increase systemcapacity. For example, the number of cells forming a cell clusterpattern can be reduced.

Utilization of digital communication techniques to communicateinformation between a sending station and a receiving station has becomeincreasingly popular in recent years. Radio communication systems, suchas cellular communication systems, are exemplary of communicationsystems which are increasingly constructed to utilize digitalcommunication techniques.

Communication channels formed between the sending and receiving stationsof a radio communication system are defined upon a portion of theelectromagnetic spectrum, the “bandwidth”, allocated to the system. Thechannels are defined, at least in part, upon carrier frequencies,“carriers”, within the allocated bandwidth. The bandwidth allocated, andavailable, to a radio communication system is usually limited. And, thecommunication capacity of the radio communication system is defined,inter alia, by, and limited by, the bandwidth allocated to the system.

In a multi-user radio communication system, such as a cellularcommunication system, communication capacity limitations sometimesprohibit additional users from utilizing the communication system as aresult of bandwidth limitations. By utilizing more efficiently thebandwidth allocated to the communication system, the communicationcapacity of the system can be increased.

The bandwidth allocated to a radio communication system can be moreefficiently utilized if digital communication techniques are used totransmit information-containing communication signals upon channelsforming links between a sending station and a receiving station.

When a communication signal is formed utilizing a digital communicationtechnique, an information signal is sometimes digitized and modulatedupon a carrier utilizing a selected modulation technique, such as, forexample, a QPSK (Quadrature Phase Shift Keying) or a GMSK (GaussianMinimum Shift Keying) technique. Use of other modulation techniques aresometimes alternately utilized. Because the information signal isdigitized, the communication signal formed therefrom can be transmittedby a sending station upon a communication channel in discrete bursts.When the communication signal is transmitted in discrete bursts, thebursts are concatenated theretogether at the receiving station.

Because communication signals can be transmitted in discrete bursts,time division multiplexing of a carrier is permitted. Two or morechannels can be defined upon a single carrier.

In at least one type of cellular communication system, a systemconstructed pursuant to the operational specification of the GlobalSystem for Mobile communications (GSM), a digital communicationtechnique is utilized. Carriers of the bandwidth allocated to thecommunication system are divided into eight time slots. Eight-way timedivision multiplexing is provided in such a communication system, andbursts of communication signal portions are transmitted between asending station and a receiving station on selected ones of such timeslots. Carrier/time slot combinations form the communication channelsupon which the communication signals are transmitted.

Standard protocols set forth in the GSM operational specification,define the structure of normal bursts communicated during time slotsdefined in the GSM system. The communication signal portions transmittedduring the time slots defined in the GSM system are divided at leastinto a data field and a training sequence field. The training sequencefield is formed of a series of bits, known to the receiving station. Thepurpose of transmitting known bits to the receiver is to allow thereceiver to equalize the channel. Typically, the signal is distortedwhen it propagates through the radio medium and the equalization allowsfor the receiver to estimate the channel impulse response, i.e., howthis distortion has affected the signal during its transmission to thereceiver.

Such training sequence bits are utilized at the receiving station tofacilitate detection of the informational content of the data fieldscommunicated together with the training sequence field.

Cellular communication systems, both those utilizing conventional analogtechniques and also those utilizing digital communication techniques,define cells throughout a geographical area encompassed by the cellularcommunication system. Collections of cells form cell clusters. In eachcell cluster, the total available bandwidth allocated to thecommunication system is utilized. In successive cell clusters, theallocated bandwidth is reused. The communication capacity, limited bythe number of channels which can be defined upon the allocatedbandwidth, is effectively increased by reusing the channels in each ofthe cell clusters.

A problem sometimes associated with reuse of the bandwidth is co-channelinterference. When communication signals are transmitted concurrently indifferent cells upon the same communication channel, suchconcurrently-transmitted signals sometimes interfere with one another;such interference is referred to as co-channel interference. Co-channelinterference makes detection of the wanted-signal received at areceiving station more difficult. If levels of co-channel interferenceare significant, the quality of the signal detection might beinadequate.

Receiving stations which receive communication signals generatedutilizing digital communication techniques sometimes include equalizercircuitry to facilitate signal detection of the informational content ofa communication signal received at the receiving station. Typically, thetraining sequence forming a portion of a communication signal isutilized by the equalizer to facilitate the detection of theinformational content of the wanted-signal received at the receivingstation.

When the receive signal received at the receiving station is formed ofboth a wanted-signal component and also an interfering-signal component,an equalizer can be constructed to jointly detect both the wanted-signalcomponent and the interfering-signal component. In such an equalizer,however, the training sequences associated with both the wanted-signalcomponent and the interfering-signal component must typically both beknown. While the training sequence associated with the wanted-signalcomponent is typically known to the receiving station, the trainingsequence associated with an interfering-signal component portion formingat least a portion of the interfering-signal component is notnecessarily and, typically is not, known to the receiving station.Without knowledge of the training sequence of the interfering-signalcomponent portion, existing receiving stations are typically unable toproperly detect and suppress such an interfering-signal componentportion of a receive signal.

A manner by which to permit a receiving station to determine better theinterfering-signal component portion of a receive signal received at thereceiving station would be advantageous. By better detecting theinterfering-signal component portion, suppression of suchinterfering-signal component portion can be better effectuated. Thereby,bandwidth reuse can be made more efficient, resulting in increasedcommunication capacities of the communication system.

It is in light of this background information related to digitalreceivers that the significant improvements of the present inventionhave evolved.

SUMMARY OF THE INVENTION

The invention claimed herein is an improvement to the invention of U.S.Pat. No. 5,933,768, commonly assigned at the time of this application;the disclosure of that patent is incorporated herein by reference. Thepresent invention advantageously provides apparatus and methods forinterference suppression in a receiver in a wireless network thatutilizes training sequences for synchronization and channel estimation,wherein the training sequence of an interfering channel overlaps thetraining sequence of a desired channel to cause degraded channelestimation, by generating channel estimates for a carrier part of areceived signal; generating residual signals where the carrier partshave been removed from the received signal; generating noise covariancematrix estimates for interferer channel estimate candidates; selectingcarrier and interferer channel estimates having the lowest energy in thecovariance matrix; and, explicitly generating the selected interfererchannel estimate. The method can be embodied in a radio receiver,particularly within an improved estimator circuit for facilitatingprocessing of a radio signal.

In one embodiment, the method further comprises the step of selectingthe carrier channel estimate with the lowest energy in the residualsignal, which step can be performed after the step of generatingresidual signals and prior to the step of generating noise covariancematrix estimates.

The method can further comprise the step of refining the carrier channelestimate. In such embodiments, the process of refining the carrierchannel estimate includes the steps of generating a residual signalwhere the interferer part has been removed; generating channel estimatesfor the carrier part of the residual signal; generating residual signalswhere the carrier and interferer parts have been removed; and, selectingthe carrier channel estimate with the lowest energy in the residualsignal. The steps of refining the carrier channel estimate can beperformed after the step of explicitly generating the selectedinterferer channel estimate.

A more complete appreciation of the present invention and the scopethereof can be obtained from the accompanying drawings which are brieflysummarized below, the following detailed description of thepresently-preferred embodiments of the invention, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a 4:12 cell reuse pattern sometimes utilized toallocate channels available to be used in a cellular communicationsystem;

FIG. 2 illustrates a cell reuse pattern, similar to that shown in FIG.1, but of a 1:3 channel reuse pattern;

FIG. 3 illustrates an exemplary frame structure of a burst into whichbits transmitted between a sending station and a receiving station of acommunication system are formatted;

FIG. 4 illustrates a functional block diagram of a model of acommunication system having a communication channel upon whichco-channel, interfering-signal components are received together with awanted-signal component at a receiving station;

FIG. 5 illustrates a functional block diagram of the apparatus to embodythe present invention and which forms a portion of the receiving stationshown in FIG. 4;

FIG. 6 illustrates another functional block diagram of an apparatus toembody the present invention and which forms a portion of the receivingstation shown in FIG. 4;

FIG. 7 illustrates a flow diagram of a prior art method for determiningthe value of a training sequence associated with an interferingcomponent portion of a received signal; and,

FIGS. 8-A, 8-B and 8-C illustrate exemplary methods in accordance withthe principles of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIG. 1, an exemplary grid pattern, shown generally at10, is illustrative of a channel allocation pattern defined in anexemplary cellular communication system. The grid pattern pictoriallyillustrates the manner by which groups of channels are reused throughouta geographical area encompassed by the system.

Hexagonally-shaped cells 12 are defined by radio base stations 14.Typically, the hexagonal pattern exists only for cell planning purposesand it is to be understood that the actual coverage provided by the basestations and antenna sites is typically not hexagonal. The radio basestations 14 form portions of cellular network infrastructure equipmentof the cellular communication system.

In the grid pattern 10 illustrated in FIG. 1, each radio base station 14defines three cells 12. Generally, a mobile terminal (not shown)positioned in one of the cells 12 transceives communication signals toand from the radio base station 14 which defines the cell in which themobile terminal is positioned. Utilization of a cellular communicationsystem is advantageous as continued communication with the mobileterminal is possible even as the mobile terminal passes throughsuccessive ones of the cells 12. Communication “handoffs” between radiobase stations 14 defining the successive ones of the cells 12 permitsuch continued communication, all without apparent interruption ofongoing communications.

As noted previously, a significant advantage of a cellular communicationsystem arises due to the ability to reuse channels defined in thebandwidth allocated to the communication system. Different groups ofchannels are assigned to be used at different ones of the cells 12. And,such channel assignments are repeated in successive groups of the cells.While typically, channel assignments of adjacent cells 12 aredissimilar, the channel assignments are repeated over the successivegroups of cells.

The grid pattern 10 shown in FIG. 1 is sometimes referred to as a “4:12”scheme. In such a scheme, the channels allocated to the cellularcommunication system are divided into twelve channel-groups. The groupsare designated in the figure by A1, A2, A3, B1, B2, B3, C1, C2, C3, D1,D2, and D3. Groups of twelve cells, such as the group 22 indicated bythe bold-face outline in the figure, are defined throughout the pattern10. The channels assigned to each cell of the twelve-cell group isassigned a different set of channels, and adjacently-positioned cellsare not assigned with the same channels. Interference between signalsgenerated in adjacently-positioned cells is thereby reduced.

As also mentioned previously, to increase the capacity of the cellularcommunication system, the cells can be re-used more frequently, albeitat the risk of an increase in the possibility that interference ofconcurrently-generated communication signals might be more likely tooccur.

FIG. 2 illustrates another exemplary grid pattern, shown generally at30, illustrative of another channel allocation pattern. The pattern 30is sometimes referred to as a “1:3” scheme. Similar to the arrangementshown in FIG. 1, hexagonally-shaped cells 12 are defined by radio basestations 14. Each base station 14 defines three cells 12, in mannersanalogous to the manners by which the base stations 14 define the cells12 in the illustration of FIG. 1.

In a 1:3 scheme, the channels of the bandwidth allocated to the cellularcommunication system are divided into three groups. In such a scheme,the channels allocated to the communication system are divided intothree channel-groups. The groups are designated in the figure by A, B,and C. Groups of three cells, such as the group 42, indicated by thebold-faced outline, are formed throughout the grid pattern 30. Each cellof the group 42 is defined with a different one of the three sets ofallocated channels. The sets of channels, again, are assigned in mannerssuch that adjacent ones of the cells are not assigned with the same setof frequency channels.

Because the allocated channels are divided into three sets of channelsrather than twelve sets of channels, a multiple-increase in channelsavailable upon which to transmit communication signals at any particularcell results. Viz., up to a four-fold increase in communication capacityis possible. However, the separation distance between cells which reusethe same set of channels is reduced, in contrast to the reuse pattern ofthe 4:12 scheme shown in FIG. 1.

Degradation levels of communication quality resulting from co-channelinterference is therefore of a potentially greater concern in a cellularcommunication system which utilizes the 1:3 scheme rather than the 4:12scheme. If increased levels of co-channel interference significantlyinterfere with communications when a 1:3 communication scheme isutilized, the benefits of the increased communication capacity permittedby the utilization of a 1:3 communication scheme might be offset byincreased levels of co-channel interference. If significant, the levelsof co-channel interference, in fact, might seriously degrade the qualityof communications, wholly obviating the possibility of increasing thecommunication capacity of the 1:3 allocation scheme.

Operation of an embodiment of the present invention provides a manner bywhich to identify a co-channel, interfering signal at a receivingstation, thereby to permit better suppression of such interferingsignal.

FIG. 3 illustrates a burst 50, exemplary of a burst into which data bitsof a communication signal are formatted in an exemplary, cellularcommunication system. The burst 50 is representative of a burstformatted during operation of transmitter apparatus of a GSM cellularcommunication system. Other manners by which a communication signal canbe formatted can similarly be represented.

As illustrated, the burst 50 includes two fields of data 52, each offifty-seven bit lengths. A training sequence 54 of a twenty-sixbit-length is positioned between the two data fields 52. Three-bitlength tails 56 are formed at the opposing ends of the burst 50. The bitvalues of the tails 56 are of zero values. The bits forming the datafields 52 contain speech or other types of source information orsignaling information. Single bit length flags 58 positioned between thedata fields 52 and the training sequence 54 are of values indicative ofthe type of information of which the fields 52 are formed. Successivebursts, or “frames” formatted therefrom, are communicated between asending and a receiving station, such as the radio base stations 14shown in FIGS. 1 and 2, and a mobile terminal to communicate informationtherebetween.

FIG. 4 illustrates an exemplary communication system, shown generally at70, representative of the cellular communication system having a cellreuse plan such as that represented by the grid pattern 30 shown in FIG.2. In the illustrated system 70, a wanted-signal transmitting station 72generates a transmit signal which is transmitted upon a communicationchannel 74 to a receiving station 76. The wanted-signal transmittingstation 72 is representative of, for example, the radio base station 14,shown in FIG. 2, which generates downlink signals for transmission to amobile terminal, here represented by, for example, the receiving station76. The station 72, conversely, can be representative of a mobileterminal when the mobile terminal is operable to transmit an uplinksignal to a radio base station.

Additional transmitting stations 78 are representative of transmittingstations which generate interfering signals which interfere with thewanted-signal generated by the transmitting station 72. A K number ofinterfering-signal transmitting stations 78 are illustrated in thefigure. Each of such transmitting stations 78 transmit interferingsignals upon the communication channel 74 and which are received by thereceiving station 76. Such transmitting stations 78 are representative,for example, of radio base stations which generate signals on the samechannel upon which the transmitting station 72 transmits signals,thereby to form co-channel interfering signals which interfere with thewanted-signal generated by the transmitting station 72.

The channels 82 upon which the signals are transmitted by thetransmitting station 72 and stations 78 can be modeled by discrete-timechannel filters containing a channel impulse response. Such filters arein the model represented by, for example, multi-tap, FIR (Finite ImpulseResponse) filters. The channels 82 illustrated in the figure arerepresentative of the channels upon which the signals are transmitted,and such channels are indicated by the designation h₀, h₁ . . . h_(k).Each of the channels 82 is of potentially differing characteristicsresponsive to the paths upon which the signals are transmitted to thereceiving station 76. Collectively, the channels 82 upon which thesignals generated by the transmitting stations 72 and 78, respectively,are transmitted form the communication channel 74.

Although the receiving station 76 is intended to receive only thewanted-signal generated by the transmitting station 72, the receivesignal actually received by the receiving station 76 is the summation ofall of the signals transmitted upon the different channels 82. Suchsummation of the different signals is represented in the figure by thesummation element 84. Additional distortion caused by white Gaussiannoise forms an additional component of the summed signal forming thereceive signal received at the receiving station 76.

FIG. 5 illustrates apparatus 85 which forms a portion of the receivingstation 76 shown in FIG. 4 in an exemplary embodiment of the presentinvention. The apparatus 85 is operable to identify and possiblysuppress one or more interfering-signal component portions of a receivesignal received at the receiving station.

While the following description of an exemplary embodiment is describedgenerally with respect to a communication system in which it isgenerally desirable to suppress interfering signal component portions,in an embodiment in which the present invention is operable in, e.g., aCDMA communication system, the received signal contains at least onewanted signal component portion and possibly a number of interferingsignal component portions.

The receive signal, subsequent to down-conversion operations performedby a down-converter (not shown), is applied to an estimator 86 by way ofline 87. The estimator 86 is operable to estimate channel impulseresponses of a communication channel upon which a wanted-signalcomponent portion and at least one interfering-signal component portionof the receive signal are transmitted. The channel impulse responses maybe estimated, for example, through the utilization of a trainingsequence, or other sequence, of the interfering signal component portionor portions. The estimator 86 generates a signal on line 88 which isapplied to an interference detector 89. This signal forms a qualitymeasure of the received-signal by being, e.g., an estimate of thequality of a wanted-signal component portion after detection. A separatequality measure is generated for each combination of wanted-signalcomponents and interfering-signal component portions that are estimatedby the estimator 86. Thereby, the estimator 86 forms a signal qualitydeterminer in which the signal formed thereat is an indication of thequality of the wanted-signal component after detection. The interferencedetector 89 is operable to selectively suppress one or moreinterfering-signal component portions of the receive signal. Theinterference suppressor may, for example, be formed of a joint detector,an interference canceler, a multi-user detector, or a subtractivedemodulator. An embodiment in which the interference suppressor isformed of a joint detector shall be described below with respect to FIG.6.

The estimator 86 which generates the signal on the line 88 is operableto estimate channel impulse responses of a channel upon which the wantedsignal component and one or more interfering signal component portionsare transmitted. The channel impulse response is determined, in oneembodiment, by utilizing a sequencer signal which permits for anestimation of the channel impulse response, such as a training sequenceof the interfering signal component portion.

FIG. 6 illustrates apparatus 92 which forms a portion of the receivingstation 76 shown in FIG. 4 in an exemplary embodiment of the presentinvention. The apparatus 92 is operable to determine one or moreinterfering-signal component portions of a receive signal received atthe receiving station. By determining the component portions of thereceive signal which are interfering-signal component portions, suchcomponent portions can be jointly detected together with a wanted-signalcomponent and thereby suppressed.

The receive signal, subsequent to down-conversion operations performedby a down-converter (not shown), is applied to an estimator 94 by way ofline 96. The estimator 94 is operable to determine the training sequenceof one or more interfering-signal component portions of the receivesignal generated on the line 96.

The estimator 94 includes a channel estimator 98. The channel estimatoris operable to estimate the channels upon which the wanted-signalcomponent and the interfering-signal component portions of the receivesignal are transmitted. That is to say, the channel estimator 98 isoperable to estimate the channels 82 shown in FIG. 4. The channelestimator 98 is provided with information, here indicated by way of line102, of the training sequence of the wanted-signal component, wanted tobe received by the receiving station of which the apparatus 92 forms aportion.

The training sequences associated with the interfering-signal componentportions are, however, not known. Only the set of training sequencesavailable to the communication system is known. Training sequencesassociated with individual ones of the interfering-signal componentportions are selected from such set. The estimator 94, of which thechannel estimator 98 forms a portion is operable to extract the trainingsequences associated with one or more interfering-signal componentportions.

The channel estimator 98 estimates groups of channels formed ofestimated channels upon which the wanted-signal component is transmittedtogether with interfering-signal component portions having associatedtherewith each of the possible training sequences.

The groups of estimates can be estimated, e.g., by joint estimation ofthe wanted-signal component and at least one interfering-signalcomponent portion. But, the groups of estimates can also be formed bycombining individual estimates of the wanted-signal component andestimates of the interfering-signal component portions, respectively.

In one embodiment, channel estimation, ĥ, is made based upon a leastmean square error estimate which is calculated using the followingequation:ĥ=(M ^(H) M)⁻¹ M ^(H) r _(T)where ^(H) denotes the Hermetian transpose, and M is the matrix definedas:M=[M₀, M₁, . . . , M_(K)]and where each M_(i), i=0,1 . . . , K is a matrix containing thetraining sequence for user i, m_(i)(n),n ε[0, N−1], in the followingway:

${M_{i} = {{\left\lfloor \begin{matrix}{m_{i}(L)} & {m_{i}\left( {L - 1} \right)} & \cdots & {m_{i}(0)} \\{m_{i}\left( {L + 1} \right)} & {m_{i}(L)} & \cdots & {m_{i}(1)} \\\cdots & \cdots & \; & \cdots \\{m_{i}\left( {N - 1} \right)} & {m_{i}\left( {N - 2} \right)} & \cdots & {m_{i}\left( {N - L - 1} \right)}\end{matrix} \right\rfloor i} = 0}},\ldots\mspace{11mu},K$

L is the memory of the channels, i.e., for each user, L+1 taps areestimated for the channel impulse responses. N is the length of thetraining sequence.

Further r_(T) is a column vector of length N−L containing the receivedsignal during the training sequence. Adopting the above definition,r_(T) can be expressed as:r _(T) =Mh+wwhere w is noise and interference that is not detected.

In the same way the received signal during the data sequence can beexpressed as:r _(D) =Dh+wwhere D is a matrix defined as:D=[D₀, D₁, . . . , D_(K)]where each D_(i), i=0,1, . . . , K, is a matrix containing the datasequence for user i,d_(i)(n),n ε[0,N−1], defined as:

${D_{i} = {{\left\lfloor \begin{matrix}{d_{i}(L)} & {d_{i}\left( {L - 1} \right)} & \ldots & {d_{i}(0)} \\{d_{i}\left( {L + 1} \right)} & {d_{i}(L)} & \ldots & {d_{i}(1)} \\\ldots & \ldots & \; & \ldots \\{d_{i}\left( {R - 1} \right)} & {d_{i}\left( {R - 2} \right)} & \ldots & {d_{i}\left( {R - L - 1} \right)}\end{matrix} \right\rfloor i} = 0}},\ldots\mspace{11mu},K$The residual interference during the training sequence is:R _(T)=(r _(T) −Mĥ)^(H)(r _(T) −Mĥ)and similarly for the data sequence:R _(D)=(r _(D) −Dĥ)^(H)(r _(D) −Dĥ)

The expected value for the residual interference for the trainingsequence is:E{R _(T)}= . . . =(N−L−(L+1)(K+1))σ² =uσ ²where σ² is the power of the non-detected signal w.

The same can be done for the data sequence:E{R _(T)}= . . . =(R−L)(Trace{(M ^(H) M)⁻¹}+1)σ² =vσ ²an estimate of the residual interference for the data sequence,{circumflex over (R)}_(D), can be done by combining the above twoequations.

${\hat{R}}_{D} = {\frac{v}{u}R_{T}}$

While the training sequence of the wanted-signal component is known, thetraining sequence of interfering-signal component portions must bedetermined.

To determine the training sequences of the interfering-signal componentportions, a joint channel estimate is performed under the assumptionthat the interfering-signal component portions have particular trainingsequences. That is to say, ĥ is calculated where M=[M₀M₁] and M₁ take onall possible training sequences. Then for all the channel estimates, avalue of the residual interference during a data-sequence portion iscalculated. An estimate of the training sequence for theinterfering-signal component portion which exhibits the lowest estimateof the residual interference during a data sequence is selected to bethe interfering-signal component which has the most degrading effectupon receiver performance.

In a second embodiment, instead of estimating the joint channelestimates fromĥ=(M ^(H) M)⁻¹ M ^(H) r _(T),and calculating the residual interference during the training sequenceasR _(T)=(r _(T) −Mĥ)^(H)(r _(T) −Mĥ),the residual interference can be calculated using a channel estimationalgorithm having lower computational complexity, especially if it isdesired to try several hypotheses of interferer training sequences. Inthe equations that follow, M=[M₀ M₁] since a single interferer istargeted.The carrier and interferer channels can be estimated directly using:ĥ=(M ^(H) M)⁻¹ M ^(H) r _(T).However, this requires the inverse of the normal equations for thecarrier and interferer, which is an (2L+2)×(2L+2) matrix. Instead, theleast-squares estimate of the carrier is first calculated, disregardingthe interferer,{circumflex over (f)} ₀=(M ₀ ^(H) M ₀)⁻ M ₀ ^(H) r _(T),and the residual signal after channel estimatione ₀ =r _(T) −M ₀ {circumflex over (f)} ₀Then, the carrier is re-estimated, now including the interfererĝ=(M ^(H) M)⁻¹ M ^(H) e ₀,and the residual interference during the training sequence is thenestimated:R _(T)=(e ₀ −Mĝ)^(H)(e ₀ −Mĝ)=e ₀ ^(H) e ₀ −e ₀ ^(H) M(M ^(H) M)⁻¹ M^(H) e ₀.Since e₀ is the residual signal after the carrier least-squaresestimation, M₀ ^(H)e₀=0, and the residual interference expressionsimplifies toR _(T) =e ₀ ^(H) e ₀ −e ₀ ^(H) M ₁ WM ₁ ^(H) e ₀,where W is a submatrix of the inverse of the partitioned matrix

$\left( {M^{H}M} \right)^{- 1} = {\begin{bmatrix}{M_{0}^{H}M_{0}} & {M_{0}^{H}M_{1}} \\{M_{1}^{H}M_{0}} & {M_{1}^{H}M_{1}}\end{bmatrix}^{- 1} = {\begin{bmatrix}X & Y \\Z & W\end{bmatrix}.}}$When the interferer training sequence has been determined, the carrierand interferer channel estimates can then be found by first calculatingthe interferer channel estimateĥ ₁ =WM ₁ ^(H) e ₀.Then, the carrier channel estimate is found by subtracting theinterferer from the received signal and then using least squaresestimationw ₁ =r _(T) −M ₁ ĥ ₁,ĥ ₀=(M ₀ ^(H) M ₀)⁻¹ M ₀ ^(H) w ₁.The joint carrier and interferer channel estimate is then

$\hat{h} = {\begin{bmatrix}{\hat{h}}_{0} \\{\hat{h}}_{1}\end{bmatrix}.}$In addition to testing all possible training sequences, differentsynchronization positions should also be considered. One way to do so isto shift the interferer training sequence symbols, inserting zeroes forpositions outside the training sequence. The preceding calculations arerepeated for all possible synchronization positions to find the residualinterference. Those skilled in the art will recognize that, in thisstep, a lot of calculations can be reused between successivesynchronization positions in the calculation of R_(T).

The foregoing calculations are based on the assumption of a singlereceiving antenna. Those skilled in the art, however, will recognizethat the calculations are very similar for multiple antennas. What ischanged is that the signals r_(T), r_(D), w and e₀, w₁, and channelestimates {circumflex over (ƒ)}₀, ĝ and ĥ become matrices where eachcolumn represents one antenna. The residual interference R_(T) and R_(D)become covariance matrices, and the determinant of these should be takenwhen selecting which training sequence and synchronization position touse.

A selector 108 is coupled to the residual interference estimator 106 byway of lines 112. The selector 108 is operable, in part, to select thetraining sequence associated with the value of residual interference atthe lowest level and to generate a signal on line 114 representative ofsuch training sequence. A signal representative of a channel estimate isalso generated upon the line 114.

The apparatus 92 further includes a joint detector 118. The jointdetector 118 is coupled to receive indications of the training sequenceand the channel estimate selected by the selector 108 and also, at leastselectively, to the line 96 upon which the receive signal is provided.The joint detector 118 is operable to detect jointly the wanted-signalcomponent having the known training sequence and the one or moreinterfering-signal component portions associated with the one or moretraining sequences selected by the selector 108. The joint detector may,for example, be implemented utilizing a Viterbi algorithm, inconventional fashion.

By jointly detecting the wanted signal component portion and the atleast one interfering-signal component portion, the degradation of thewanted-signal component portion caused by the interfering-signalcomponent portion is substantially reduced, i.e., a suppression of theinterfering-signal component portion out of the wanted-signal componentportion is achieved.

In the illustrated embodiment, the apparatus 92 further includes asingle-channel detector 128. And, in such an embodiment, the channelestimator 98 is further operable to estimate a wanted-signal channelupon which the wanted-signal component of the receive signal isestimated to be transmitted, all without regard to anyinterfering-signal component portion.

The residual interference estimator 106 is further operable to calculatethe residual interference of such an estimated channel, and the selector108 is further operable to select amongst the channel estimatesincluding such single-channel estimate. If a determination is made bythe selector 108 that the single-channel estimate exhibits the lowestlevel of residual interference, the selector 108 generates a controlsignal on line 134 which controls a switch position of a switch element136.

The switch element 136 alternately connects the line 96 to either thejoint detector 118 or the single-channel determiner 128. When theresidual interference value of the single-channel estimate is of thelowest value, the selector 108 causes the switch element 136 to bepositioned to interconnect line 96 with the detector 128. When anotherof the channel estimates exhibits lower levels of residual interference,the selector 108 causes the switch position of the switch element 136 tobe positioned to interconnect the line 96 with the joint detector 118.In such manner, the apparatus detects the receive signal jointly orsingly, as appropriate.

FIG. 7 illustrates a flow diagram of a prior art method 200 fordetermining the value of a training sequence associated with aninterfering component portion of a received signal. The method 200 isoperable to determine the value of a training sequence associated withan interfering component portion of a receive signal. The receive signalis formed of a wanted-signal component and at least oneinterfering-signal component portion.

First, and as indicated by the block 202, groups of channel estimatesare generated responsive to indications of the receive signal. Channelestimates are estimative of channels upon which components of thereceive signal are transmitted to the receiver. Then, and as indicatedby the block 204, values of residual interference are generated for eachof the groups of channel estimates. And, as indicated by the block 206,a selection is made of the group of channel estimates which exhibits thelowest value of residual interference. Such levels of residualinterference are indicative of the interference-signal componentportion. The training sequences associated with the channel estimatesare determined to be the value of the training sequence.

Turning now to FIGS. 8-A, 8-B and 8-C, illustrated are exemplary methodsin accordance with the principles of the invention relating tointerference suppression in a radio receiver in a wireless network thatutilizes training sequences for synchronization and channel estimation,wherein the training sequence of an interfering channel overlaps thetraining sequence of a desired channel to cause degraded channelestimation.

In FIG. 8-A, all combinations of carrier and interferer channelestimates are tested. The method includes the steps of: generatingchannel estimates for a carrier part of a received signal (810);generating residual signals where the carrier parts have been removedfrom the received signal (820); generating noise covariance matrixestimates for interferer channel estimate candidates (830); selectingcarrier and interferer channel estimates having the lowest energy in thecovariance matrix (840); and, explicitly generating the selectedinterferer channel estimate (850).

FIG. 8-B, interferer channel estimates are tested for only a selectedcarrier channel estimate. The method further includes the step ofselecting the carrier channel estimate with the lowest energy in theresidual signal (825). In the embodiment illustrated in FIG. 8-B, thisstep is performed after the step of generating residual signals andprior to the step of generating noise covariance matrix estimates.

In the further embodiment illustrated in FIG. 8-C, the methodillustrated in FIG. 8-B is improved by refining the carrier channelestimation. The carrier channel estimate can be refined through thesteps of: generating a residual signal where the interferer part hasbeen removed (860); generating channel estimates for the carrier part ofthe residual signal (870); generating residual signals where the carrierand interferer parts have been removed (880); and, selecting the carrierchannel estimate with the lowest energy in the residual signal (890). Inthe embodiment illustrated in FIG. 8-C, these steps are performedimmediately after the step of explicitly generating the selectedinterferer channel estimate.

Operation of an embodiment of the present invention permits aninterfering-signal component portion to be detected even without priorknowledge of a training sequence associated with such portion. A receivesignal received at a receiver is analyzed, first to determine a trainingsequence associated with the interfering-signal component portion. And,a determination is made of the manner by which to recover best thewanted-signal component of the receive signal. A selection is madeeither to recover the wanted-signal component by joint detection or bydetection of merely the wanted-signal component of the receive signal.

When embodied in receiving station portions of a cellular communicationsystem, such as the receiver portion of a mobile terminal or thereceiver portion of a radio base station, better suppression ofco-channel interference is facilitated. Because co-channel interferenceis better able to be suppressed, channels defined in this system can bereused in a more efficient manner, such as the 1:3 cell reuse patternshown in FIG. 2.

Although the present invention and its exemplary embodiments areprimarily explained considering a TDMA communication system, it couldequally well be implemented in communication systems that utilize otherkinds of access schemes, such as, for example, a slotted CDMA system,where every time slot/frequency is divided into at least two trafficchannels, e.g., at least two users. The user separation within the timeslot/frequency is in such a case achieved by a code separation, i.e.,each user in the same time slot/frequency is assigned a user-specificspreading code. In such a case, a wanted signal component is susceptibleto both co-channel and intra-cell interference. If an embodiment of theinvention is implemented in, e.g., such a system, it should beappreciated that what earlier has been referred to as interfering-signalcomponent portions actually can be any mixture of interfering-signalsand useful signals, at least for the uplink case, where detection of allcomponents are desirable.

The previous descriptions are of preferred examples for implementing theinvention, and the scope of the invention should not necessarily belimited by this description. The scope of the present invention isdefined by the following claims.

1. A method of interference suppression in a receiver in a wirelessnetwork that utilizes training sequences for synchronization and channelestimation, wherein the training sequence of an interfering channeloverlaps the training sequence of a desired channel to cause degradedchannel estimation, said method comprising the steps of: generatingchannel estimates for a carrier part of a received signal: generatingresidual signals where the carrier parts have been removed from thereceived signal; generating noise covariance matrix estimates forinterferer channel estimate candidates; selecting carrier and interfererchannel estimates having the lowest energy in the covariance matrix;explicitly generating the selected interferer channel estimate;selecting the carrier channel estimate with the lowest energy in theresidual signal; and refining the carrier channel estimate by:generating residual signals where the interferer parts have beenremoved; generating channel estimates for the carrier part of theresidual signal; generating residual signals where the carrier andinterferer parts have been removed; and, selecting the carrier channelestimate with the lowest energy in the residual signal.
 2. The methodrecited in claim 1, wherein said steps for refining the carrier channelestimate are performed after said step of explicitly generating theselected interferer channel estimate.
 3. An apparatus for interferencesuppression in a receiver in a wireless network that utilizes trainingsequences for synchronization and channel estimation, wherein thetraining sequence of an interfering channel overlaps the trainingsequence of a desired channel to cause degraded channel estimation, saidapparatus comprising: means for generating channel estimates for acarrier part of a received signal; means for generating residual signalswhere the carrier parts have been removed from the received signal;means for generating noise covariance matrix estimates for interfererchannel estimate candidates; means for selecting carrier and interfererchannel estimates having the lowest energy in the covariance matrix;and, means for explicitly generating the selected interferer channelestimate; and means for refining the carrier channel estimate, saidrefining means comprising: means for generating residual signals wherethe interferer parts have been removed; means for generating channelestimates for the carrier part of the residual signal; means forgenerating residual signals where the carrier and interferer parts havebeen removed; and, means for selecting the carrier channel estimate withthe lowest energy in the residual signal.
 4. In a radio receiveroperable to receive a radio signal, the received signal formed of awanted-signal component portion and at least one interfering-signalcomponent portion, an improved estimator circuit for facilitatingprocessing of the radio signal, said estimator circuit comprising:processing circuitry, said processing circuitry operable to: generatechannel estimated for a carrier part of said received signal; generateresidual signals where the carrier parts have been removed from thereceived signal; generate noise covariance matrix estimates forinterferer channel estimate candidates; select carrier and interfererchannel estimates having the lowest energy in the covariance matrix;and, explicitly generate the selected interferer channel estimate;select the carrier channel estimate with the lowest energy in theresidual signal; and refine the carrier channel estimate; wherein saidprocessing circuitry refines the carrier channel estimate by: generatingresidual signals where the interferer parts have been removed;generating channel estimates for the carrier part of the residualsignal; generating residual signals where the carrier and interfererparts have been removed; and, selecting the carrier channel estimatewith the lowest energy in the residual signal.
 5. The estimator circuitrecited in claim 4, wherein said processing circuitry refines thecarrier channel estimate after explicitly generating the selectedinterferer channel estimate.